Supply of a liquid-metal target in X-ray generation

ABSTRACT

Closed-loop circulation for providing liquid metal to an interaction region at which an electron beam is to impact upon the liquid metal to produce X-rays is presented. In a method, the pressure of the liquid metal is raised to at least 10 bar using a high-pressure pump. The pressurized liquid metal is then conducted to a nozzle and ejected into a vacuum chamber in the form of a spatially continuous jet. After passage through the vacuum chamber, the liquid metal is collected in a collection reservoir, and the pressure of the liquid metal is raised to an inlet pressure, e.g. using a primer pump, suitable for the inlet of the high-pressure pump. Also, a corresponding circulation system and an X-ray source provided with such circulation system.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. application Ser. No.13/262,499, filed on Nov. 15, 2011, which is a U.S. National Stage ofInternational Application No. PCT/EP2009/002464, filed on Apr. 3, 2009.The entire contents of each of U.S. application Ser. No. 13/262,499 andInternational Application No. PCT/EP2009/002464 are hereby incorporatedherein by reference in their entirety.

TECHNICAL FIELD

The present disclosure generally relates to liquid-metal targets,particularly for use in electron impact X-ray sources.

TECHNICAL BACKGROUND

X-rays have traditionally been generated by letting an electron beamimpact upon a solid anode target. However, thermal effects in the anodelimit the performance of the X-ray source. One way of mitigating theproblems relating to overheating of the solid anode target has been touse a rotating solid anode.

A second conventional approach is to direct the electron beam towards aliquid anode target, such as in the form of freely falling droplet orstream. U.S. Pat. No. 4,953,191 shows an X-ray source using as its anodea falling stream of liquid gallium, which is substantially flat andtypically has a tangential velocity of about 2.0 m/s. In view ofoverheating, the power of an X-ray source of this type will be limitedby the mass of gallium transported per unit time by the flowing stream.Spatial localization is typically a desirable property of the source,and neither thickening the stream nor increasing its speed are availableas ways to increase the mass flow.

More recently, it has been proposed to use a liquid jet as electrontarget in X-ray generation. For example, WO 02/11499 discloses a methodand apparatus for generating X-ray or EUV radiation using a liquid jetas a target for an electron beam. Such X-ray sources may comprise agrounded jet of liquid metal adapted to act as anode, i.e., as targetfor the electron beam. By virtue of its regenerative nature, such jet ofliquid metal can withstand strong electron beam impact; as a comparison,the propagation speed of the jet can be similar to or higher than thetangential speed of a conventional rotating anode. Although only afraction of the energy carried by the electron beam is converted intoX-ray energy—which leads to a considerable excess heat generation—theseliquid jet X-ray sources are nevertheless characterized by excellentbrightness, which brings benefits relating to exposure duration, spatialresolution and new imaging methods, such as phase-contrast imaging.

However, it has been a challenge to devise a liquid metal jet X-raysource that can operate for extended periods of time withoutinterruptions for maintenance. For example, in previous X-ray sources ofthis kind, an operator has been required to halt the generation ofX-rays in order to change or refill a target supply container, or tochange or refill containers for pressurized propellant gas that is usedfor generating the target jet.

SUMMARY

A general object of the present invention is to provide a closed-loopsystem for supply of liquid metal to an interaction region where anelectron beam impacts the liquid metal to generate X-rays. It isenvisaged that the present invention will provide prolonged operationaltimes before X-ray generation must be interrupted for maintenance orservice.

By way of introduction, the context and some challenges relating tosystems for supply of a liquid metal jet will be briefly discussed.

An X-ray source of the mentioned type includes an electron gun and asystem for providing a steady jet of liquid metal inside a vacuumchamber. The metal used is preferably one having a comparably lowmelting point, such as indium, gallium, tin, lead, bismuth or an alloythereof. The electron gun may function by the principle of cold-fieldemission, thermal-field emission, thermionic emission and the like. Thesystem for providing the electron-impact target, i.e. the liquid-metaljet, may include a heater and/or a cooler, a pressurizing means, a jetnozzle and a reservoir for collecting the liquid metal at the end of thejet. X-ray radiation is generated in the impact region as a result ofthe interaction of the electrons and the liquid-metal target. A windowhaving suitable transmission characteristics allows the X rays thusgenerated to leave the low-pressure chamber. To allow continuousoperation of the device, it is desirable to recover the liquid metaldownstream from the interaction region and to reuse the recovered liquidmetal in a closed-loop fashion.

On a technological level, supply of the liquid-metal jet in aclosed-loop manner has been found to entail potential weaknesses. Theuniformity of the jet, in terms of speed, shape and thickness, may bedissatisfactory due to pressure variations caused by, e.g., the movementof pump pistons, intermittent mechanical obstruction by solidcontaminant contained in the liquid, fluctuations of liquid levels invarious parts of the system, and even loss of suction capacity of thepump. Under some conditions, the jet may also become spatiallydiscontinuous and break up into segments or droplets before a pointwhere the electron beam impacts upon the jet, which would lead tocomplications with respect to the X-ray generation.

Leakage of metal is another potential challenge of a closed-loop systemfor providing the liquid-metal jet. The result of leakage may be thatmetal is permanently lost to the exterior of the system, but alsoincludes the case of metal solidifying in parts of the system that areinaccessible. For example, a mist of suspended liquid droplets may beproduced at the exit of the nozzle and at the point where the liquid jetimpacts the liquid contained in the collecting receptacle. If such mist,which is relatively mobile, deposits on an inner wall of thelow-pressure chamber, it will be more or less permanently lost from thecirculation. Moreover, if the low-pressure chamber is a pumped vacuumchamber adapted to continuously evacuate gaseous or gas-suspendedparticles therein, an amount of liquid may be expelled from the systemthis way. Finally, seals, piping and pumps are all sources of potentialleakage of liquid and therefore weak points of the circulation loop.From a user's point of view, leakage may release potentially toxic gas,necessitate expensive replenishment of liquid, shorten maintenanceintervals (particularly the periods between cleaning of the X-ray outputwindow), deteriorate performance by spoiling the vacuum conditions, andgenerally make operation of the associated X-ray source more difficultfor continuous periods of time. These challenges are addressed andmitigated by the present invention.

Proposed herein, in accordance with a first aspect of the invention, istherefore a method for closed-loop supply of liquid metal to aninteraction region, in which an electron beam is to impact upon theliquid metal to generate X rays, particularly by bremsstrahlung orcharacteristic line emission. The method comprises the following steps:

-   -   The pressure of liquid metal contained in a first portion of a        closed-loop circulation system is raised to at least 10 bar,        preferably at least 50 bar or more, using a high-pressure pump.    -   The pressurized liquid metal is conducted to a nozzle. Although        any conduction through a conduit will entail some, possibly        negligible under the circumstances, loss of pressure, the        pressurized liquid metal reaches the nozzle at a pressure still        above 10 bar, preferably above 50 bar.    -   The liquid metal is ejected from the nozzle into a vacuum        chamber, in which the interaction region is located, for        generating a liquid metal jet.    -   The ejected liquid metal is collected in a collection reservoir        after passage through the interaction region.    -   The pressure of the collected liquid metal is raised to a        suction side pressure (inlet pressure) for the high-pressure        pump, in a second portion of the closed-loop circulation system        located between the collection reservoir and the high-pressure        pump in the flow direction (i.e., during normal operation of the        system, liquid metal flows from the collection reservoir towards        the high-pressure pump). The inlet pressure for the        high-pressure pump is at least 0.1 bar, preferably at least 0.2        bar, in order to provide reliable and stable operation of the        high-pressure pump.        The steps are then typically repeated continuously—that is, the        liquid metal at the inlet pressure is again fed to the        high-pressure pump which again pressurizes it to at least 10 bar        etc.—so that the supply of a liquid metal jet to the interaction        region is effected in a continuous, closed-loop fashion.

The high-pressure pump used for pressurizing the liquid metal in thehigh-pressure portion of the circulation loop comprises dedicatedhigh-pressure elements, so as to allow a considerable pressuredifference between its suction side and discharge side. The pump may inparticular be of a sealless type, in order to reduce the risk ofleakage. Dynamic seals, such as sliding seals or rotary seals, areindeed known to be particularly sensitive to leaks at high pressures.This could cause both pressure fluctuations (since the tightness of adynamic seal may vary with its actual position) and a steady escape ofliquid. In the context of the present invention, sealless pumps, such asdiaphragm pumps, are preferred over pumps having dynamic seals.

The step of raising the pressure of the collected liquid metal to asufficient inlet pressure ensures good operating conditions for thehigh-pressure pump, enabling it to supply a steady high pressure to thenozzle and protecting it from losing its suction capacity. Preferably,the propagation speed of the liquid-metal jet through the vacuum chamberis at least 10 m/s. The liquid metal jet is generated so that the jetwill be spatially continuous at the point where the electron beamimpacts the metal jet. It is noted, however, that breakup of the jetbeyond the point where the electron beam impacts the target jet isgenerally acceptable since such late breakup of the jet does not affectthe generation of X-rays to any relevant extent; the possibility of suchbreakup does not influence the dimensioning of the nozzle.

The raising of the liquid metal pressure from vacuum pressure up to theinlet pressure may be achieved passively using the gravitational fieldby collecting the ejected liquid in a reservoir having such placementthat a column of liquid metal at its bottom sup-plies a hydrostaticpressure of suitable magnitude to the suction side of the high-pressurepump, at least during operation. Alternatively, the reservoir may beessentially flat but be connected to the high-pressure pump via aliquid-filled (during operation) duct extending down in thegravitational field. Hence, in relation to the collection reservoir, thehigh-pressure pump should in this case be located lower in thegravitational field, and at least part of the connection between these(in the flow direction) should contain liquid metal during steady-stateoperation that provides a sufficient inlet pressure for thehigh-pressure pump.

Sufficient inlet pressure may also be provided actively, e.g. by meansof a primer pump arranged between the collection reservoir and thehigh-pressure pump, for providing pressurized liquid metal to thesuction side of the latter.

It will also be understood that the inlet pressure for the high-pressurepump can be obtained using a combination of the gravitational field anda primer pump.

As explained earlier, the X-ray generation process produces aconsiderable amount of excess heat. A high temperature may acceleratecorrosion and other types of deterioration of the system. In order toremove the excess heat from the liquid metal there may be provided acooler, such as a refrigerating coil thermally connected to thecollection reservoir, adapted to discharge excess heat delivered by theelectron beam to the outside the circulation system. Such cooler may beoperated at a rate proportionate to the actual intensity of the electronbeam with an aim of maintaining the liquid metal contained in thecirculation system at a moderate set-point temperature, such as slightlyabove the melting point for the metal at issue. It should be understood,however, that the temperature of the liquid metal will vary through thedifferent portions of the closed-loop circulation system.

More generally, a temperature control may be applied. Apart fromremoving excess heat generated by electron bombardment to avoidcorrosion and overheating of sensitive components in the system, theremay be a need for heating the liquid metal in other portions of thesystem. Heating may be required if a metal with a high melting point isused and the heat power supplied by the electron beam is not sufficientto maintain the metal in its liquid state throughout the system. As aparticular inconvenience, if the temperature falls below a criticallevel, splashes of liquid metal hitting portions of the inner wall ofthe collection reservoir may solidify and be lost from the liquidcirculation loop of the system. Heating may also be required if a largeoutward heat flow takes place during operation, e.g., if it turns out tobe difficult to thermally insulate some section of the system. It shouldalso be understood that heating for start-up may be required if themetal used is not liquid at typical ambient temperatures.

The interaction region, in which the X-ray generation takes place, isprovided in a vacuum chamber. For X-ray generation in the context of thepresent invention, a pressure of at most 10⁻³ mbar is preferably sought.The low pressure requirement is primarily due to the electron beamsystem. For some elaborate electron beam systems, still higher vacuumpressures could potentially be acceptable.

In an advantageous embodiment, the method further includes damping ofpressure pulses which the high-pressure pump may excite in the liquidmetal. It is known that many displacement-type pumps—including single-or multi-headed piston pumps and diaphragm pumps—do not expel thepressurized medium in a continuous manner, and this is an obstacle toproviding a uniform liquid-metal jet. Damping of pressure pulses can beachieved using a pulsation damper of suitable type. For example, thedamper may be a diaphragm accumulator, a bladder accumulator or a pistonaccumulator. A diaphragm accumulator may comprise a volume of gas thatcan be compressed by a pressure pulse in the liquid metal. The gas isthen typically enclosed behind a diaphragm with its other side incontact with the liquid metal contained in the system. The gas may alsobe provided at a closed top end of a vertical pipe partially filled withthe liquid metal and in free communication with the rest of the system.Alternatively, the damper can be a compliant section, adapted toelastically flex and absorb pressure pulses, of the duct connecting thehigh-pressure pump and the nozzle in the flow direction. Other kinds ofpressure pulse dampers may also be conceived.

Many X-ray sources comprising a liquid target suffer to some extent fromcontamination by debris emanating from the target material, in thepresent case metal, which is distributed in the vacuum chamber in theform of vapor, mist and splashes. In order to reduce the production ofsuch debris, an advantageous embodiment of the inventive method providescollection of the liquid metal by letting the ejected jet impinge on aslanting surface. The slanting surface deflects any splashes away fromthe vacuum chamber and towards the collection reservoir.

In some implementations of the inventive system, the liquid metal may bepassed through one or more filters during its circulation in the system.For example, a relatively coarse filter may be arranged between thecollection reservoir and the high-pressure pump in the normal flowdirection, and a relatively fine filter may be arranged between thehigh-pressure pump and the nozzle in the normal flow direction. Thecoarse and the fine filter may be used separately or in combination.Embodiments including filtering of the liquid metal are advantageous inso far as solid contaminants are captured and can be removed from thecirculation before they cause damage to other parts of the system.

According to a second aspect of the invention, there is provided acirculation system for supplying a liquid-metal electron target forX-ray generation. An X-ray source comprising such circulation system isalso provided.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will now be described in detailwith reference to the accompanying drawings, on which:

FIG. 1 is a flow chart of a method for closed-loop supply of liquidmetal according to an embodiment of the invention;

FIG. 2 is a diagrammatic overview of the inventive circulation systemfor providing a liquid metal target for X-ray generation by electronbeam impact;

FIG. 3 schematically shows an alternative embodiment of the circulationsystem outlined in FIG. 2;

FIG. 4 schematically illustrates a first implementation of the liquidmetal collection reservoir;

FIG. 5 schematically illustrates a second implementation of the liquidmetal collection reservoir; and

FIG. 6 schematically shows an X-ray source comprising a closed-loopcirculation system for liquid metal according to the present invention.

On the drawings, similar parts or details are generally indicated bysimilar reference numerals throughout.

DETAILED DESCRIPTION OF EMBODIMENTS

A method 100 for closed-loop supply of liquid metal, in accordance witha currently preferred embodiment of the invention, will now be describedwith reference to FIG. 1. For clarity and simplicity of this disclosure,the method will be described in terms of ‘steps’. It is emphasized that‘steps’ are not necessarily processes that are delimited in time orseparate from each other, and more than one ‘step’ may be performed atthe same time in a parallel fashion. The intended outlook of thisdisclosure is that the ‘steps’ represent the different treatments that aliquid undergoes during its loop through a circulation system adapted toperform the method.

In step 110, the pressure of the liquid metal is raised to a highpressure. The high pressure should be sufficient in order for the liquidmetal jet to obtain a high propagation speed in the vacuum chamber onceinjected from the nozzle. Typically, the high pressure will be at least10 bar, preferably at least 50 bar and up to more than 100 bar. Withreference to FIG. 3, the liquid metal that is being pressurized isaccommodated in a closed-loop circulation system 300 comprising ahigh-pressure pump 312, preferably a diaphragm pump or otherhigh-pressure pump. The pump constitutes a transition to a first portion310 (high pressure portion) of the circulation system 300.

In step 112, the pressurized liquid metal is conducted through the highpressure portion 310 of the system 300 towards a nozzle 332. Hence, thehigh pressure portion 310 of the circulation system is located betweenthe high-pressure pump 312 and the nozzle 332. The pressurized liquidmetal will lose some pressure during its transportation from the pump312 to the nozzle 332 in step 112, but the pressure is kept sufficientlyhigh in order for the metal to reach the nozzle at a sufficiently highpressure of at least 10 bar, and typically at least 50 bar. In thisembodiment, the pressurized liquid metal is passed through an optionalfine filter 316 before it reaches the nozzle 332. The fine filter 316may be provided in the form of a stainless-steel sieve or severalconsecutive sieves.

Depending on the nature of the high-pressure pump 312, pressure pulsespropagating in the pressurized liquid metal may occur as a side effectof the pressurization. Such pressure pulses may be damped in an optionalthird step 114 using a damper 318 provided in the first portion of thesystem 310, before or after the fine filter 316. The damper may comprisea membrane having an amount of gas (such as compressed N₂) enclosedbehind it. On the arrival of a pressure pulse at the damper 318, theliquid-side pressure increases, the membrane extends and compresses thegas. As the pressure sinks back, the gas expands again to its formervolume. In effect, any residual pulse propagating onwards past thedamper 318 towards the nozzle 332 has a reduced amplitude. For clarityof FIG. 3, a connection line has been drawn from the first portion 310of the circulation system and the damper 318. Nevertheless, a damper ofa similar kind may be arranged directly, in-line, in the first portion310 of the system and/or have the form of an enlarged duct segmenthaving as wall a flexible membrane supported by a compressed gas volume,as described above. In embodiments where a stronger damping is required,multiple dampers may be provided in the first portion 310.

In step 116, the liquid metal is ejected by the nozzle 332 into a vacuumchamber 330. The steady (spatially continuous) liquid-metal jet may thenbe used as a target for an electron beam (not shown) in this chamber330. The electron beam impacts the liquid metal in an interaction region(not shown), and part of the electron beam energy is converted intoX-rays. The nozzle orifice to be used has such shape and dimensions thatthe ejected liquid metal assumes the form of a physically continuousjet. The jet will tend to relax into a state of lower surface energy andthereby typically change its shape. As explained above, jet breakup intoa spray, droplets or other kinds of discontinuous portions beyond thepoint where the electron beam impacts the metal jet can be tolerated,since any influence on the X-ray generation of such late breakup isnegligible. Likewise, gravitation will have very little influence on theshape of the jet, which does not have to be parallel with thegravitational field. Suitably, the nozzle is a sapphire plate or rubyplate, through which a cylindrical or conical hole has been provided.The diameter of the hole may be of the same order as the thickness ofthe plate. The shape of the nozzle should minimize production ofliquid-metal mist, which may contaminate the vacuum chamber 330. Thedesired cross-section of the jet, with regard to optimal X-raygeneration conditions, may be taken as a starting point in dimensioningthe nozzle. In the preferred implementation, the pressure of the liquidmetal at the nozzle is about 100 bar. The nozzle has an ejection orificethat is 100 μm in diameter, which is similar to the wall thickness ofthe sapphire nozzle. A suitable electron beam for impact upon the liquidmetal jet has a focus of about 20 μm diameter, and a power of about 200W. Having read and understood the present disclosure, the skilled personwill rely on routine calculations and experiments to find suitableparameters.

In step 118, the ejected liquid metal is collected in a collectionreservoir 334. As already discussed above, the liquid metal may need tobe cooled in order to avoid overheating of the system. Typically, thetemperature of the liquid metal in the reservoir should preferably bemaintained at a temperature slightly above the melting temperature, andmay vary as a function of the melting point of the metal, the kind ofmetal used, the thermal insulation of the system 300 and the heatresistance of its components. Cooling may be effected by aheat-conducting circuit 336 which expels the excess heat H outside thesystem. If the heat conduction between the reservoir 334 and the outsideof the system is large, a passive heat-conducting circuit may providesufficient cooling. If more powerful cooling is required, one may useactive refrigeration including compression of a cooling medium. In apreferred embodiment, the cooling is effected using water-cooledaluminum blocks which are clamped to the reservoir and/or the conduits.

In the interest of reducing contamination of the vacuum chamber 330,care is taken in order that the collection of the liquid-metal jetproduces as little debris as possible. It has been found that thecollection of the liquid metal progresses in a regular, non-chaotic andnot very splashing fashion if the jet is conducted into the collectionreservoir 334 via a slanting surface. The slanting surface may be aninner wall of the reservoir 334 or a plate arranged in proximity of theliquid-metal surface during steady-state operation of the circulationsystem 300. Best debris-preventing results have been achieved using aslanting surface at 45 degrees or less with respect to the metal jet.The slanting surface may be as steep as 10 degrees or even less.Moreover, the liquid-metal surface may be covered by a screen comprisingan entry aperture for the jet; such screen is adapted to capture mistand splashes and prevent these from contaminating the vacuum chamber330.

As a further step 120, in order to provide good working conditions forthe high-pressure pump 312, the pressure of the collected liquid metalis raised to an inlet pressure for the high-pressure pump. This steptakes place in a second portion 320 of the system 300 located betweenthe collection reservoir 334 and the suction side of the high-pressurepump 312. In this embodiment, the pressure rise is effected actively bymeans of a primer pump 322. For a high-pressure pump in the form of adiaphragm metering pump as used in the preferred implementation, theinlet pressure to the high-pressure pump should be at least 0.1 bar,preferably at least 0.2 bar.

A coarse mechanical filter 324, such as a stainless-steel filter, mayoptionally be provided upstream of the primer pump 322 to protect itfrom potentially harmful solid contaminants.

As the liquid metal is again pressurized to at least 10 bar by thehigh-pressure pump 312, it has completed a loop in the system 300. Thesteps described above will now be repeated in a continuous fashion, sothat a steady supply of liquid metal to the interaction region isachieved in the closed-loop system.

A preferred metal for use in the method above is an alloy comprising 95wt % of Ga and 5 wt % of In, having a melting point of 25° C. and afreezing point of about 16° C. Other conceivable metals for use in themethod are Galinstan™, which is an alloy comprising 68.5 wt % Ga, 21.5wt % In and 10 wt % Sn, having a melting and freezing point of about−19° C.; an alloy comprising 66 wt % In and 34 wt % Bi, having a meltingand freezing point of about 72° C.; and pure In having a melting andfreezing point of about 157° C.

In the following, an apparatus according to the present invention willbe described in detail, with reference to the accompanying drawings andparticularly FIGS. 3, 4 and 5 thereof.

An apparatus according to the present invention in the form of aclosed-loop circulation system 300 for liquid metal is schematicallyshown in FIG. 3 as a block diagram. The apparatus comprises a vacuumchamber 330 in which an electron beam (not shown) is to impact upon theliquid metal target jet in order to produce X-ray radiation bybremsstrahlung and characteristic line emission. A nozzle 332 isprovided at the vacuum chamber for generating the liquid metal targetjet. The nozzle is operatively connected to a high-pressure pump 312through a pipe or conduit system. The portion 310 of the closed-loopcirculation system between the nozzle and the high-pressure pumpconstitutes a high pressure portion (first portion) in which pressurizedliquid metal is transported towards the nozzle 332 in order to beejected as the liquid metal target jet. In the high pressure portion 310of the circulation system, there may optionally be provided a damper318. The purpose of such damper 318 is to reduce any pressure pulses inthe pipe system caused by the high-pressure pump 312, in order for thegeneration of the liquid metal target jet at the nozzle 332 to be smoothand continuous. In addition, the high pressure portion 310 of thecirculation system may comprise a filter, such as the fine filterindicated at 316, for removing any fine particles that may otherwiseclog or hamper the nozzle 332.

The high-pressure pump 312 works from a second portion 320 of theclosed-loop circulation system, in which the pressure of the liquidmetal is considerably lower than in the high pressure portion 310. Atthe inlet side of the high-pressure pump, the pressure of the liquidmetal should have a suitable inlet pressure. A primer pump 322 isprovided in the second portion of the circulation system for raising thepressure of the liquid metal from the low pressure present at the vacuumchamber to a suitable inlet pressure. Typically, the inlet pressure atthe inlet side of the high-pressure pump is at least 0.1 bar, morepreferably at least 0.2 bar. The second portion of the pipe system 320connects the high-pressure pump 312 to a collection reservoir 334. Afterhaving been ejected in the form of a target jet by the nozzle 332, andafter having passed the vacuum chamber, the liquid metal is collected inthe collection reservoir 332 for further recirculation, and the primerpump 322 raises the pressure to a suitable inlet pressure for thehigh-pressure pump 312. In alternative embodiments, as shown in FIG. 2,the primer pump 322 can be dispensed with if other means for raising thepressure to a suitable inlet pressure for the high-pressure pump isprovided. For example, a sufficient inlet pressure for the high-pressurepump 212 can be obtained by arranging the high-pressure pump 212 at suchposition that a column of liquid metal is present above the pump 212. Inother words, the high-pressure pump 212 is located lower in thegravitational field than the collection reservoir 234 and thehydrostatic pressure caused by the column of liquid metal provides asufficient inlet pressure.

An optional filter, such as a coarse filter 324, may be provided in thefluid path of the liquid metal between the collection reservoir 334 andthe high-pressure pump 312 for removing particulate contamination in theliquid metal.

When an electron beam impacts the liquid metal target jet inside thevacuum chamber, a large amount of heat is deposited in the liquid metal.In order to remove excess heat, a cooling system 336 may be provided inconnection with the collection reservoir. In the preferred embodiment,the cooling system comprises one or more water cooled blocks clamped tothe pipe system close to the collection reservoir. The water cooledblocks can be aluminum blocks having channels for the cooling water.

Some examples of suitable metals are given above. The temperaturecontrol of the metal within the closed-loop circulation system ispreferably such that the temperature of the metal is maintained slightlyabove the melting point for the metal at issue. Of course, thetemperature of the metal immediately after having its interaction withthe electron beam will be considerably higher until the cooling inconnection with the collection reservoir has had its effect. However,for the other portions of the circulation system, the temperature of themetal is best kept moderate. One reason for keeping the metaltemperature at moderate levels is that the metal may become aggressiveto the piping material and other parts of the system at hightemperatures.

It should also be noted that some metals used in connection with thepresent invention may require that heat is supplied at some portions inorder to maintain the metal liquid at all times during operation of thesystem. This may, for example, mean that a heater can be added at thehigh-pressure portion of the circulation system. Moreover, supply ofheat is often required during start-up of the system when the metal usedis not liquid at ambient (room) temperatures.

FIGS. 4 and 5 are two examples of how the liquid metal collection couldbe effected. It should be noted that FIGS. 4 and 5 are highly schematicand only illustrate the general principles. In FIGS. 4 and 5, the liquidmetal jet is schematically indicated by the downwards-pointing arrows.

As described above, care should be taken when collecting the liquidmetal jet in order to avoid or reduce formation of debris (splashes,mist, etc.) in the vacuum chamber. To this end, there is provided aslanting surface that the liquid metal jet hits before the liquid metalis collected. In one embodiment, the slanting surface as well as thecollection reservoir is part of the pipe system, as schematically shownin FIG. 4. In the embodiment shown in FIG. 4, the outlet from the vacuumchamber is designed such that the liquid metal jet exits the vacuumchamber through an opening 410, continues some distance through astraight portion of an exit pipe 415, and eventually hits a slantingportion 420 of the piping. Thereby, the risk of splashes, mist or otherdebris finding its way back into the vacuum chamber 330 is considerablyreduced. In order to further reduce the risk of debris in the vacuumchamber, the exit opening 410 from the vacuum chamber may be providedwith an aperture in order to reduce the physical opening between thevacuum chamber and the piping. The collection reservoir 334 is in thiscase constituted by the lower part of the exit pipe from the vacuumchamber, as indicated in FIG. 4.

As an alternative, the straight portion 415 of the piping may be locatedwithin the general region of the vacuum chamber, as schematically shownin FIG. 5. It is also conceivable that the collection reservoir has theform of a container. If the piping itself or the container lacks anatural slanting surface for the liquid metal jet to hit after itspassage through the vacuum chamber, the slanting surface may be providedin the form of a plate or a cone or similar, which is also schematicallyshown in FIG. 5.

The present invention also relates to an X-ray source which comprises aclosed-loop circulation system for liquid metal according to the above.Such X-ray source is schematically shown in FIG. 6. The X-ray source 600comprises, in addition to the closed-loop liquid metal circulationsystem 300, an electron source 610 for generating an electron beam 612.The electron beam is to interact with the liquid metal jet to generate,preferably by bremsstrahlung and characteristic line emission, X-rayradiation. The generated X-ray radiation propagates as indicated by thearrow 614 towards an X-ray window 616, through which the X-ray radiationexits the vacuum chamber 330. In FIG. 6, the vacuum chamber is shown toenclose both the circulation system 300 and the electron beam source610. However, it may be preferred to have e.g. circulation pumps andsome of the piping for the liquid metal outside the vacuum chamber.Also, the electron source 610 is only shown very schematically, and willin any implementation normally have some parts located outside of thevacuum chamber.

The invention claimed is:
 1. A method for closed-loop supply of liquidmetal to an interaction region, in which an electron beam is to impactupon the liquid metal to generate X rays, comprising the steps of:raising a pressure of the liquid metal in a first portion of aclosed-loop circulation system to at least 10 bar using a high-pressurepump; conducting the liquid metal at a pressure of at least 10 bar to anozzle; ejecting the liquid metal from the nozzle into a vacuum chamber,in which the interaction region is located, for generating a liquidmetal jet propagating through the vacuum chamber at 10 m/s or more;collecting the ejected liquid metal in a collection reservoir; raising apressure of the collected liquid metal to an inlet pressure suitable forthe high-pressure pump, said inlet pressure being at least 0.1 bar; andrepeating the above steps to effect supply of liquid metal to theinteraction region in a closed loop, wherein a collection reservoir withan aperture and a slanting surface that the metal jet hits is used tocollect the ejected liquid metal in order to reduce contamination of thevacuum chamber by liquid-metal splashes, mist and the like from thecollection reservoir, wherein the liquid metal is passed through atleast one filter on its passage from the collection reservoir to thehigh-pressure pump and/or on its passage from the high-pressure pump tothe nozzle, and wherein the at least one filter is a coarse filterdisposed between the collection reservoir to the high-pressure pumpand/or the at least one filter is a fine filter disposed between thehigh-pressure pump and the nozzle, and the method comprises: i) passingthe liquid metal through the coarse filter on its passage from thecollection reservoir to the high-pressure pump; and/or ii) passing theliquid metal through the fine filter on its passage from thehigh-pressure pump to the nozzle.
 2. The method of claim 1, wherein theinlet pressure is at least 0.2 bar.
 3. The method of claim 1, whereinthe liquid metal jet is spatially continuous during its propagationthrough the vacuum chamber from the nozzle up to a point where theelectron beam is to impact upon the liquid metal jet.
 4. The method ofclaim 1, further comprising the step of damping pressure pulses in theliquid metal in said first portion of the closed-loop circulationsystem.
 5. The method of claim 1, wherein the step of raising thepressure using the high-pressure pump comprises raising the pressure toat least 50 bar.
 6. The method of claim 1, wherein the step of raisingthe pressure of the collected liquid metal to the inlet pressurecomprises locating the high-pressure pump lower in the gravitationalfield than the reservoir.
 7. The method of claim 1, wherein the step ofraising the pressure of the collected liquid metal to the inlet pressurecomprises using a primer pump.
 8. A closed-loop circulation system forsupply of liquid metal to an interaction region in which an electronbeam is to impact upon the liquid metal to generate X-rays, comprising:a high-pressure pump connected to a first side of a high-pressureportion of the circulation system; a nozzle connected to a second sideof the high-pressure portion of the circulation system; a vacuum chamberfor receiving liquid metal ejected in the form of a jet from the nozzle,wherein said jet propagates through the vacuum chamber at 10 m/s ormore; a collection reservoir for collecting the liquid metal afterpassing the vacuum chamber; and means, located between the vacuumchamber and the high-pressure pump in the flow direction of the liquidmetal, for raising a pressure of the liquid metal to at least 0.1 bar inorder to provide an inlet pressure for the high-pressure pump, whereinthe pressure of the liquid metal in the high-pressure portion of thecirculation system is at least 10 bar, wherein the collection reservoiris provided with an aperture and a slanting surface that the metal jethits in order to reduce contamination of the vacuum chamber byliquid-metal splashes, mist and the like from the collection reservoir,wherein the system comprises at least one filter for removing particularcontamination from the circulating liquid metal, and wherein the atleast one filter comprises: i) a fine filter arranged between thehigh-pressure pump and the nozzle in the flow direction of the liquidmetal; and/or ii) a coarse filter arranged between the collectionreservoir and the high-pressure pump in the flow direction of the liquidmetal.
 9. The circulation system of claim 8, wherein the inlet pressureis at least 0.2 bar.
 10. The circulation system of claim 8, furthercomprising a damper for dampening of pressure pulses in the liquid metalcaused by the high-pressure pump.
 11. The circulation system of claim 8,wherein the aperture of the collection reservoir is an aperture in anexit opening from the vacuum chamber, said aperture being designed andstructured to reduce risk of debris entering the vacuum chamber from thecollection reservoir.
 12. The circulation system of claim 8, wherein themeans for raising the pressure of the liquid metal to at least 0.1 barin order to provide an inlet pressure for the high-pressure pump has theform of a primer pump.
 13. An X-ray source comprising an electron sourceand a closed-loop circulation system according to claim 8.